Either start a new game, or start the 'arrival' cutscene in the cutscene section of your guerilla handbook. Somewhere in the cutscene you and Dan drive past a house with a red faction graffiti on it that's about to get raided by EDF soldiers. Listen closely to the first scream after they kick down the door.

That, ladies and gentlemen,is the infamous wilhelm scream, used throughout films and video games.